Because of recent concerns about environmental issues such as global warming, etc., there is a need to develop highly efficient, clean energy sources. Fuel cells attract attention as a candidate for such energy sources. Fuel cells directly generate electricity by supplying electrodes separated by an electrolyte with fuel (e.g. hydrogen gas, methanol) and an oxidizing agent (e.g. oxygen), respectively, and oxidizing the fuel on one electrode while reducing the oxidizing agent on the other electrode. One of the most important components among the above fuel cell materials is the electrolyte which can form an electrolyte membrane separating the fuel and the oxidizing agent. Various electrolyte membranes have been developed and, particularly in recent years, polymer electrolytes that include a polymer compound containing a proton conductive functional group (e.g. a sulfonate group) have been actively developed. Such polymer electrolytes can be used not only for polymer electrolyte fuel cells but also as materials for electrochemical elements such as humidity sensors, gas detectors, and electrochromic display elements, for example. In particular, polymer electrolyte fuel cells among the aforementioned applications of the polymer electrolytes are expected to be one of key new energy technologies. For example, polymer electrolyte fuel cells that include an electrolyte membrane formed from a proton conductive functional group-containing polymer compound are characterized in that they allow operation at low temperatures, reduction in size and weight, and the like. Such polymer electrolyte fuel cells are considered for application to movable vehicles such as automobiles, household cogeneration systems, small portable devices for consumer use, and the like.
In the 1950s, styrene-based cation exchange membranes were developed as electrolyte membranes for polymer electrolyte fuel cells. However, such membranes are poor in stability in a fuel cell operating environment and thus have not been able to produce fuel cells having sufficiently long life. On the other hand, perfluorocarbon sulfonic acid membranes, typically represented by Nafion®, are widely considered as electrolyte membranes having practical stability. Perfluorocarbon sulfonic acid membranes are high in proton conductivity and are thought to be excellent in chemical stability such as acid resistance and oxidation resistance. However, the materials of Nafion® cost high and the production process thereof is complicated, so that Nafion has the disadvantage of very high costs. Besides, it is pointed out that Nafion deteriorates due to hydrogen peroxide generated by the reactions at the electrodes and hydroxy radicals which are by-products of the reactions. Furthermore, the structure of Nafion limits the introduction of sulfonate groups, which are proton conductive groups, thereinto.
In this context, the development of hydrocarbon-based electrolyte membranes is again desired. This is because hydrocarbon-based electrolyte membranes are characterized in that they are easily modified to have various chemical structures, and thus allow a wide range of adjustment to introduce proton conductive groups (e.g. sulfonate groups), and that they are relatively easy to combine with other materials and to crosslink, etc.
In a recent example, many sulfonate groups are introduced into an electrolyte membrane to improve the proton conductivity of the membrane. Still, such a membrane is disadvantageous for use as an electrolyte membrane for fuel cells because the membrane greatly swells when it is wet, and the strength of the membrane is deteriorated as a result of repeating wet and dry cycles. Thus, a rigid structure is being attempted to be introduced into an electrolyte membrane to increase the strength of the membrane.
An example of such rigid structures is a benzophenone structure. However, it is usually difficult for polymers containing such a rigid structure to have an increased molecular weight because then they present a solubility problem in reaction solvents. Patent Literature 1 teaches that the carbonyl group of the benzophenone structure is converted into an alkyl ether during polymerization to improve solubility, and that after the molecular weight is increased, the resulting polymer is subjected to acid treatment so that the alkyl ether is re-converted into a carbonyl group. Moreover, Patent Literature 2 discloses the synthesis of a polyether ether ketone containing a rigid benzophenone structure. In order to provide a high molecular weight polymer, a t-butyl group is introduced into part of the aromatic rings. As mentioned above, it is usually difficult to increase the molecular weight of a polymer with a benzophenone structure, which requires improved techniques, such as increasing solubility. However, although these polymers contain an ether bond and have relatively high solubility, they disadvantageously easily deteriorate when they are used for electrolyte membranes for fuel cells.
Patent Literature 3 (see Reference Synthesis Example 3) mentions as examples of polymers having no ether bond but containing a benzophenone structure, random copolymers partially including poly(4,4′-benzophenone). Still, the copolymers thus obtained have a number average molecular weight as low as about 12000. This means that it is difficult for such copolymers to have a higher molecular weight. Moreover, this literature teaches that such a polymer is sulfonated, but this method is usually incapable of introducing a sulfonate group into the poly(4,4′-benzophenone) structure.
Non-Patent Literature 1 discloses the synthesis of poly(2,5-benzophenone) which has the same composition as poly(4,4′-benzophenone), and the comparison of the properties between the polymers. As taught in this literature, since the polymers have the same composition but differ in linking position, the poly(4,4′-benzophenone) has a higher crystallinity and a lower solubility than the poly(2,5-benzophenone), and therefore the poly(4,4′-benzophenone) is difficult to synthesize and cannot be synthesized by the same method. In order to produce poly(4,4′-benzophenone), this literature provides an improved technique in which the carbonyl groups of monomers are converted into imino groups and the resulting monomers are polymerized, followed by hydrolysis to re-convert the imino groups into carbonyl groups. As mentioned above, poly(4,4′-benzophenone) is difficult to directly synthesize.